When drilling, a top drive or tower drill is used as drive unit for rotation and handling of the drill string when this is vertical in the centre of the well. The tower drill was introduced on the market around 1980 to replace the Kelly and rotary table, which up to that point had been the only available drive system. The main object of the tower drill is that the momentum of rotation is provided to the drill string at the top, instead of through a rotation pipe. The system can rotate and pump drilling mud continuously during drilling due to application of hydraulic or electric motors arranged above the pipe. The tower drill is frequently used on land-based rigs. The tower drill is connected to the top of the drill string directly via standard threads used in this industry.
These connections comprise a male and a corresponding female threaded section. Both API and NC threads are used. Further specifications are available in the API standards.
Some machines utilize a torque wrench to tighten and loosen the connection between the drilling machine and drill string. For example in the following connections:                Between machinery and intermediate sub (between the tower drill and the intermediate sub/cross-over sub)        Between the intermediate sub and an internal blow out preventer (IBOP),        Between machinery and IBOP        Between IBOPs if more than one are installed, and        Between IBOP and saver sub.        
As alternative to the torque wrench, the drilling machinery can be provided with a back-up grabber. In such an arrangement, the drive unit of the machinery is used as power source to provide the required torque when tightening or loosening the connections described above.
For drilling machinery provided with a back-up grabber or back-up tong, it is of importance that the connection between machinery and the elements described above remains intact when the machinery is released from the drill string. The interlocking between the parts is normally provided by a set of flanges with an internal conical slot and fitted split inner rings. When the flanges are connected by a number of bolts, the inner ring is forced towards the connection units and a locking by friction is provided. This causes a secure locking, but the installation and dismantling is time-consuming and the risk of injury is high.